exfat: optimize exfat_zeroed_cluster()

Replace part of exfat_zeroed_cluster() with exfat_update_bhs().
And remove exfat_sync_bhs().

Signed-off-by: Tetsuhiro Kohada <kohada.t2@gmail.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>